The present invention relates to a nuclear fuel assembly and, more particularly, to a nuclear fuel assembly suitable for use for a boiling water reactor.
The reactor core of a boiling water reactor is charged with a plurality of nuclear fuel assemblies each of which is constituted by a channel box, lower tie plate, upper tie plate and a multiplicity of fuel rods. The fuel rods arranged in the form of a bundle are held at their upper and lower ends by the upper and lower tie plates. The bundle of the fuel rods is disposed in a channel box secured to the upper tie plate. Each fuel rod is charged with a multiplicity of fuel pellets (UO.sub.2 pellets). In each fuel assembly, there are several fuel rods having UO.sub.2 pellets which contain gadolinea as a burnable poison. There also are two water rods disposed in the central region of the nuclear fuel assembly.
In general, a boiling water reactor exhibits a void distribution in the vertical or axial direction. Due to the variation of void reactivity along the axis, the boiling water reactor shows such a power distribution that the peak of the power is shifted to the lower side along the axis.
In order to attain a flat axial power distribution by obviating the axially downward shifting of the power peaking, it has been proposed to use a fuel assembly having different degrees of enrichment at the upper and lower regions thereof. One of such a fuel assembly is disclosed in the specification of the U.S. Pat. No. 4,229,258. In this fuel assembly, some of the fuel rods arranged in the peripheral region thereof have different degrees of enrichment at their upper and lower regions. More specifically, the upper region of each of such fuel rod has an enrichment which is about 15% higher than that in the lower region thereof.
In recent years, various studies have been made for the development of fuel assemblies suitable for higher burn-up, i.e., fuel assemblies which can be burnt up to a high degree. Such a fuel is obtained by arranging fuel rods rich in fissile material, i.e. fuel rods having high enrichment, in the vicinity of the channel box having a high density of thermal neutron flux. Japanese Patent Laid-Open No. 26292/1983 discloses a fuel assembly which can be burnt up to a high degree in accordance with the theory disclosed in U.S. Pat. No. 4,229,258.